This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In amphibians pituitary gonadotropin induces ovarian follicles to produce a maturation-inducing steroid (MIS). It is believed the MIS is progesterone that binds to a cell surface receptor to initiate oocyte maturation. In addition, the follicular progestin may also be involved in the regulation of oocyte ovulation. Oocyte maturation and ovulation are required for normal fertilization in amphibians. Little is known about the biochemical characteristics of the progesterone receptors. In the present study we hypothesized that different types of progesterone receptors were involved in the processes of oocyte maturation and ovulation in amphibians. Ovarian tissue samples were isolated from adult Xenopus laevis and incubated with human chorionic gonadotropin in modified Barth solution for 0, 2, 4, 8, and 18 h. After the incubation, ovulated and nonovulated oocytes were collected, homogenized, centrifuged to obtain protein samples. The proteins were separated by electrophoresis in a sodium dodecyl polyacrylamide gel and then transferred to a polyvinylidene difluoride membrane. The membrane was incubated with a human progesterone receptor antibody. Results demonstrated the presence of four different progesterone receptor isoforms of approximately 68, 28,23, and 20 KDa in Xenopus ovarian follicles. Varying expression of the receptor isoforms was observed in the follicles at different developmental stages (stage I to VI) and after different incubation times with gonadotropin. These proteins were not present in other tissue types including liver, stomach, heart, and muscle of Xenopus laevis. This study is the first to report the presence of four progesterone receptor isoforms in the amphibian ovarian follicles. Roles of the progesterone receptor isoforms in oocyte maturation and ovulation of Xenopus laevis are presently unknown and merit further investigation.